The following publications are of interest as explanation of the background of this invention.
American Water Works Association (AWWA) Handbook, McGraw-Hill© 1971, 1990, 1999, 2010.
“A Research Needs Assessment for the Capture, Utilization and Disposal of Carbon Dioxide from Fossil Fuel-Fired Power Plants,” U.S. Department of Energy, DOE/ER-30194, July 1993.
Photocatalysis. Kirk-Othmer Encyclopedia of Chemical Technology vol. 19, 5th ed. 2006, pp. 73-106, Jean-Marie Herrmann.
Serpone, N. et al, “Heterogeneous Photocatalized Oxidation of Phenol, Cresols and Fluorophenols in TiO2 Aqueous Suspensions,” Photosensitive Metal-Organic Systems; American Chemical Society, Advances in Chemistry Series (ACS), pp. 281-313, 1993.
Brune, D. E. et al, “Microalgal Biomass for Greenhouse Gas Reductions: Potential for Replacement of Fossil Fuels and Animal Feeds,” Journal of Environmental Engineering, pp. 1136-1144, November 2009.
“Combining Municipal Services” ©2005, www.krebsandsislerlp.com.
The large scale conversion of wastewaters and salt water to potable fresh water is being accomplished today in combinations of biological, chemical, filtration and ultraviolet radiation processes. However, the costs to reach clean water quality are so high that such methods are usually not attempted to obtain the level of potable water purity. For example, in municipal sewage wastewater reclamation, one of the largest treatment systems in the United States releases its process effluent into rivers at a published purity of only 95% to 98%, and after aerobic and anaerobic bacterial digestion, a sludge remainder of several hundred tons per day still must be hauled and disposed. Many books, papers and patents discuss the processes, technologies and the problems related to such water conditioning. A comprehensive overview is the AWWA Handbook mentioned above.
Many wastewaters (also salt water) include some or most of the nutrients needed for microalgae growth. In sewage treatment, typically a multitude of aerobic and anaerobic micro-organisms “activate” the sewage and grow biomass while their metabolism also promotes the outgassing of methane, hydrogen sulfide, ammonia and some CO2. It is thought that, if such activated sewage sludge secondary treatment can be supplemented with sufficient and well-balanced nutrients, continuous lighting and large amounts of CO2, and if preferred species of algae which require CO2 for growth are selected, then anaerobic digestion will be suppressed with a reduction in the outgassing of methane, hydrogen sulfide and ammonia, and the autotrophic growth of cyanobacteria microalgae that require CO2 should dominate.
There are many locations in the United States where large amounts of CO2 are continuously available at little or no cost in mixtures of refinery offgases and the flue gases of hydrocarbon fueled power plants. However, the CO2 content is usually only 5 to 15% and such gases are considered unsuitable for use in microalgae growth processes without separation to better than 90% purity. Known separation processes are expensive. One source of better than 90% purity CO2 has evolved from the development of high efficiency O2/CO2 combustion for hydrocarbon and biomass fuels, as in U.S. Pat. No. 6,907,845, wherein the byproduct of steam electric generation from a condensing boiler is a stream of relatively pure CO2. In this energy production concept, the costs of separating oxygen for fuel combustion and all of the costs of separating and recovering CO2 are offset by virtually complete recycling of waste heat. Such improvement reduces fuel costs and can produce low cost electric power for use in an associated photosynthesis-photocatalysis water treatment system. Moreover, dry biomass produced from CO2 being recycled in such a system can be used as fuel.
Photocatalysis in water treatment has the potential to destroy all or nearly all organic and inorganic compounds including toxic ones. Once mineralized, the elements are absorbed by algae in photosynthesis. By mineralized, for the purposes of this application is meant the conversion or transformation of organic and inorganic compounds by oxidation and/or reduction to mineral substances that can then be metabolized by microalgae. The above-cited references: (1) Photocatalysis: Kirk-Othmer Encyclopedia of Chemical Technology, and (2) Serpone et al, show the extent of research over many years. Although scaling up has not been promising so far, it is felt that photosynthesis and photocatalysis, acting effectively together, should be able to process the total mineral content of wastewater and other waters using the visible light spectrum. To date, it has been demonstrated that certain TiO2 materials have excellent activity among photocatalysts. The recent development of a carbon-doped TiO2 by Kronos International, Inc., see U.S. Pat. No. 7,615,512, which is effective to “degrade contaminants and pollutants in liquids and gases,” is promising as a photocatalyst that may be able to significantly reduce the time required for the complete mineralization of chemical compounds in water treatment.
Concepts for the use of photosynthesis to dispose of CO2 by growing algae in shallow open ponds have heretofore been fraught with problems, as discussed at length in a 1993 U.S. Department of Energy report referenced above. When thousands of tons of CO2 need to be disposed daily, thousands of acres of land area must be utilized. One producer of the cyanobacteria Spirulina has grown this premium health food additive in Hawaii since 1985 at a rate of one ton per day in 80 acres of ponds. CO2 is 27% carbon by molecular weight, and cultured Spirulina is 50% carbon. If grown at the above rate, to convert 1,000 tons of CO2 per day (yielding 270 tons of carbon), 43,200 acres of ponds would be required to produce 540 tons of algae per day, assuming no CO2 outgassing losses. Moreover, there are other problems, such as the diurnal and seasonal lighting constraints on growth in open ponds, variable weather limitations, CO2 outgassing losses and large water losses by evaporation. It is felt that biogrowth efficiency must be increased many times to practically convert CO2 to biomass at a large scale. Furthermore, there have been concerns over the presence of some species of blue-green algae in state-of-the-art water treatment. Odor and taste problems are attributed to byproducts of metabolism in the growth of some algae (see AWWA Handbook 1990 ed, pp. 101-103, 151, 769). The 1999 AWWA Handbook notes that three species of cyanobacteria microalgae, also known as blue-green algae, may be toxic (Ch. 2.13). However, other species including Chlorella and Spirulina are food nutrients, and Spirulina is favored due to its filamentous growth which makes it easy to harvest; moreover, its high (50%) carbon content renders it favorable for direct use for fuel when dried. Nutritionally, Spirulina is 60% protein, 20% carbohydrate and a rich source of A, B and E vitamins.
As a consequence, better solutions to purification of water, use of available CO2 and growth of useful microalgae biomass continue to be sought.